All in the title, Game log :

[18:06:07] [Client thread/INFO]: [CHAT] Usage: /worldborder warning <time|distance>
[18:06:10] [Client thread/INFO]: [CHAT] An unknown error occurred while attempting to perform this command
[18:06:10] [Server thread/ERROR]: Couldn't process command: 'worldborder warning time'
java.lang.ArrayIndexOutOfBoundsException: 2
at dg.a(SourceFile:120) ~[1.8-pre1.jar:?]
at ab.a(SourceFile:80) [1.8-pre1.jar:?]
at ab.a(SourceFile:63) [1.8-pre1.jar:?]
at rj.d(SourceFile:635) [1.8-pre1.jar:?]
at rj.a(SourceFile:622) [1.8-pre1.jar:?]
at lu.a(SourceFile:37) [1.8-pre1.jar:?]
at lu.a(SourceFile:9) [1.8-pre1.jar:?]
at ih.run(SourceFile:13) [1.8-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.7.0_45]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.7.0_45]
at net.minecraft.server.MinecraftServer.z(SourceFile:576) [1.8-pre1.jar:?]
at net.minecraft.server.MinecraftServer.y(SourceFile:531) [1.8-pre1.jar:?]
at cyf.y(SourceFile:155) [1.8-pre1.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:447) [1.8-pre1.jar:?]
at java.lang.Thread.run(Unknown Source) [?:1.7.0_45]
[18:06:13] [Server thread/ERROR]: Couldn't process command: 'worldborder warning distance'
java.lang.ArrayIndexOutOfBoundsException: 2
at dg.a(SourceFile:120) ~[1.8-pre1.jar:?]
at ab.a(SourceFile:80) [1.8-pre1.jar:?]
at ab.a(SourceFile:63) [1.8-pre1.jar:?]
at rj.d(SourceFile:635) [1.8-pre1.jar:?]
at rj.a(SourceFile:622) [1.8-pre1.jar:?]
at lu.a(SourceFile:37) [1.8-pre1.jar:?]
at lu.a(SourceFile:9) [1.8-pre1.jar:?]
at ih.run(SourceFile:13) [1.8-pre1.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.7.0_45]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.7.0_45]
at net.minecraft.server.MinecraftServer.z(SourceFile:576) [1.8-pre1.jar:?]
at net.minecraft.server.MinecraftServer.y(SourceFile:531) [1.8-pre1.jar:?]
at cyf.y(SourceFile:155) [1.8-pre1.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:447) [1.8-pre1.jar:?]
at java.lang.Thread.run(Unknown Source) [?:1.7.0_45]
[18:06:14] [Client thread/INFO]: [CHAT] An unknown error occurred while attempting to perform this command